fix: use correct quickstart URL

[devjgm]

This change is 

[codecov]

Codecov Report

Merging #1339 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1339      +/-   ##
==========================================
+ Coverage   94.64%   94.64%   +<.01%     
==========================================
  Files         172      173       +1     
  Lines       13630    13639       +9     
==========================================
+ Hits        12900    12909       +9     
  Misses        730      730

Impacted Files	Coverage Δ
google/cloud/spanner/client.cc	97.22% <0%> (-0.7%)	⬇️
...on_tests/rpc_failure_threshold_integration_test.cc	85.55% <0%> (-0.16%)	⬇️
google/cloud/spanner/value.h	92.8% <0%> (-0.11%)	⬇️
google/cloud/spanner/keys.h	100% <0%> (ø)	⬆️
google/cloud/spanner/internal/transaction_impl.h	100% <0%> (ø)	⬆️
...cloud/spanner/internal/partial_result_set_resume.h	100% <0%> (ø)	⬆️
.../spanner/benchmarks/multiple_rows_cpu_benchmark.cc	91.97% <0%> (ø)	⬆️
google/cloud/spanner/internal/spanner_stub.cc	66.66% <0%> (ø)	⬆️
google/cloud/spanner/transaction.h	100% <0%> (ø)	⬆️
google/cloud/spanner/instance_admin_connection.h	100% <0%> (ø)
... and 2 more

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